The present application relates to a horizontal-electric field type liquid crystal display panel, particularly relates to a FFS (Fringe Field Switching) mode liquid crystal display panel in which an upper electrode or a lower electrode is formed over all sub-pixels in a display area as well as operates as a common electrode.
As the liquid crystal display panel is characterized by light-weight, thin and low power consumption as compared with a CRT (cathode ray tube), it is used for many electronic devices for display. The liquid crystal display panel displays images by changing the alignment of liquid crystal molecules aligned in a given orientation and changing the transmittance of light in a liquid crystal layer. The liquid crystal display panel includes a reflective type in which external light is incident on the liquid crystal layer, reflected at a reflector, transmitted through the liquid crystal again and emitted, a transmissive type in which incident light from a backlight device transmits through the liquid crystal layer and a semi-transmissive type having the both properties. There are a monochrome display type and a color display type in the liquid crystal display panel. The color of a pixel in the color-display type liquid crystal display panel is fixed by mixture of light transmitted through respective sub-pixels having color filters of three primary colors of light which are R (red), G (green) and B (blue) individually.
As a method of applying an electric field to the liquid crystal layer of the liquid crystal display panel, there are a vertical-electric field type and a horizontal-electric field type. In the vertical-electric field liquid crystal display panel, an approximately vertical electric field is applied to liquid crystal molecules by a pair of electrodes arranged opposite to each other so as to sandwich the liquid crystal layer. As the vertical-electric field liquid type crystal display panels, a TN (Twisted Nematic) mode, a VA (Vertical Alignment) mode, a MVA (Multi-domain Vertical Alignment) mode, an ECB (Electrically Controlled Birefringence) mode and the like are known. In the horizontal-electric field liquid crystal display panel, a pair of electrodes is provided at one of a pair of substrates arranged sandwiching the liquid crystal layer on the liquid crystal layer side so as to be insulated from each other, in which an approximately horizontal electric field is applied to liquid crystal molecules. As the horizontal-electric field type liquid crystal display panel, an IPS (In-Plane Switching) mode in which a pair of electrodes do not overlap each other in planar view and a FFS mode in which they overlap each other. As the horizontal-electric field type liquid crystal display panel has an advantage of obtaining a wide viewing angle, it becomes widely used in recent years.
As the pair of electrodes is provided in the same layer in the IPS mode liquid crystal display panel, there are problems in which liquid crystal molecules positioned on the upper side of the pixel electrode are not sufficiently driven, and that, a low aperture ratio and a low transmittance may arise. Accordingly, the FFS mode liquid crystal display panel disclosed in JP-A-2009-036800 (Patent Document 1) and JP-A-2008-180928 (Patent Document 2) has been developed. In the FFS mode liquid crystal display panel disclosed in Patent Document 1, a lower electrode operates as a common electrode and the common electrode is formed so as to be separated in each sub-pixel. A common line is formed in the same layer parallel to a scanning line connected to a gate electrode of a thin film transistor TFT for driving the pixel electrode so that the common electrode partially overlaps with the common line, thereby performing wiring of the common electrode with respect to each sub-pixel.
As described above, the common electrode in the FFS mode liquid crystal display panel disclosed in Patent Document 1 overlaps with the common line, therefore, a level difference occurs and affects the upper electrodes, as a result, alignment of liquid crystal molecules is disordered at a part corresponding to the level difference. Accordingly, in the liquid crystal display panel disclosed in Patent Document 2, an interlayer resin film also called a planarization film is formed on TFTs and lower electrodes, the lower electrodes operating as pixel electrodes are formed on the interlayer resin film, an inter-electrode insulating film and an upper electrode operating as a common electrode are formed in order. The common electrode is formed over all sub-pixels in the display area, the liquid crystal display panel is characterized by higher aperture ratio and higher contrast as compared with the liquid crystal display panel disclosed in Patent Document 1.